Cells of Nervous System

Define Nervous Tissue

The body’s nervous system is defined as the system responsible for transmitting signals from the body to the brain and vice versa. The nervous system consists of the brain, spinal cord, and nerves. The nerves are the part of the nervous system which transmits signals from various body parts to the brain and the brain’s response to the signals perceived by it to the respective body parts. The nerves are made of cells called neurons. The example nervous system is divided into the Central Nervous System (CNS), the brain and the spinal cord, and the Peripheral Nervous System (PNS), which consists of the nerves and neurons. The PNS connects the various body parts to the CNS and transmits the response of the CNS to the body parts. In this article, we will be studying in detail the PNS, especially the nervous system cells, which are neurons. 

Neurons and Their Functions

The cells of the nervous system are called neurons. Neurons emerge from the brain right up to various body parts. The primary function of the neurons is to receive signals from the various body parts, transmit the signal to the brain, and then transmit the brain’s response to the respective body part. Signals can be received through smell, sight, touch, taste, and hearing.

A straightforward example can explain the function of neurons. When you touch a hot surface with your hand, the neurons of your skin will transmit the sense of hotness from the skin to the brain; and the brain will then transmit another signal through neurons to the hand to withdraw it from the hot surface. All this data is transmitted through the neurons.

Structure of Neurons

The neuron has three essential parts- cell body, axon and dendrites. We will read about each cell part in detail.

Cell Body

The cell of the nervous system is just like any other cell. It has a nucleus which consists of the nucleolus. It contains other cytoplasmic cell organelles, like ribosomes, Golgi apparatus, mitochondria, etc. However, the neuron lacks centrioles, which are responsible for cell division. Hence, a neuron cannot perform cell division, and as a result, the neurons in our body are limited and, once damaged, cannot be repaired or multiplied.

Dendrites

Dendrites are extensions of the cell body. They are branched-like structures that emerge from the cell body. They are the cytoplasmic extension of the cell body, which means they are similar to the cell body. They are short and branched to increase the surface area of receiving signals. The number of dendrites on each cell is different as each neuron has a different purpose or signal to transmit. 

Axon

Axon is another protrusion of the cell body. Axons are usually elongated and transmit signals away from the cell body, hence also called efferent processes. The axons terminate into short branches called axon terminals. Axons are typically protected by a covering of fatty substances called the myelin sheath. The axon area in between two myelin sheaths is called Nodes of Ranvier. 

Neuroglia

Neuroglia is another type of nerve cell. It is different from neurons because its primary function is to provide nutrition to the neurons and not transmit signals to and from the body to the brain.   

Conditions Affecting Neurons

Several conditions affect the neurons which control our muscles and movements. These neurons are called motor neurons. Some of the most common motor neuron conditions are explained below-

• Amyotrophic Lateral Sclerosis (ALS)

ALS is a motor neuron disease. It affects both upper motor neurons (neurons in the brain) and lowers motor neurons (neurons in the spinal cord). In this motor neuron condition, the patient’s neurons start to deteriorate over time. As mentioned above, neurons do not have centrioles responsible for cell division. Hence the damaged neutrons cannot repair or reproduce. This causes the patient suffering from ASL to lose muscle control, and over time the patients are unable to walk, chew, talk, and slowly lose control of all the muscles. The onset of ASL is between 40-60, and patients can usually live up to 3-5 years. ASL is not a genetic condition and can happen to anyone.

• Primary Lateral Sclerosis (PLS)

PLS is another motor neuron disease that affects the upper motor neurons. This condition affects the limbs and their movement primarily. Sometimes chewing and talking are also hampered. The onset of PLS also begins from 40-50 years of age. However, unlike ALS, patients don’t die from PLS.

• Progressive Bulbar Palsy (PBP)

This is a condition that precedes ALS. This condition affects the motor neurons in the brain stem, which is present at the base of the brain. This condition causes the patient to slur, have difficulties in speaking and chewing. Usually, PBP progresses to ALS; hence PBP is also known as a form of ALS.

1. Progressive Muscular Atrophy 

Atrophy of muscles is the reduction in the size of the tissues of the muscles. Atrophy causes the muscles to go weak. In progressive muscle progressive atrophy disorder, the muscles begin to atrophy over time due to damaged motor neurons. This hampers movement of limbs, talking, walking, etc. This condition is not as common as ALS, and it may or may not be genetic.

• Spinal Muscular Atrophy

This condition affects the lower motor neurons. It is a genetic condition that affects the SMN1 gene, which makes a particular protein that protects the neuron. Spinal muscular atrophy causes weakness in the lower limbs and trunk. It is classified into several types depending upon the onset of the condition.

Type1- In this condition, the onset is between 0 to 6 months. Children suffering from this cannot sit on their own.

Type 2- Here, the onset is between 6 months to 12 months.

Type3- Onset is between 2 years to 17 years. Kids may have trouble walking, running, talking, and climbing stairs.

Type 4- Here, the onset is after the age of 30 years. It affects the muscles of the lower limbs. 

Conclusion 

The body’s nervous system consists of the brain, spinal cord, and nerves. The nerves are made of neurons. The primary function of neurons is to transmit signals from body parts to the brain and vice versa. The neuron consists of the cell body, which includes the nucleus and other cell organelles; dendrites which are projections of the cell body in the form of branches; and the axon, which is one long projection of the cell body. The axon ends in branched-like axon terminals. The axon is covered by a fatty substance called myelin sheath for protection against damage. The neuron does not contain centrioles which are responsible for cell division. So if the neurons are damaged, they cannot be repaired back.

Motor neurons are the neurons responsible for the movement of the body. Several conditions affect the motor neurons like Amyotrophic Lateral Sclerosis, primary lateral sclerosis, progressive muscular atrophy, progressive bulbar palsy, etc. These conditions may or may not be genetic and affect the body’s muscles, thereby affecting movements like walking, talking, speaking, etc.